Various thermal transfer recording methods are known in the art. Among others, a method for forming various full-color images has been proposed. In this method, a thermal transfer sheet comprising dye layers formed by holding, by a suitable binder, dyes as recording materials for dye sublimation transfer on a substrate such as a polyester film is provided, and the sublimable dyes are thermally transferred from the thermal transfer sheet onto a thermal transfer image-receiving sheet comprising a dye receptive layer provided on an object dyeable with a sublimable dye, for example, paper or plastic film to form a full-color image. In this case, a large number of color dots of three or four colors with the quantity of heat being regulated are transferred by heating by means of a thermal head as heating means in a printer onto a receptive layer in the thermal transfer image-receiving sheet to reproduce a full color of an original by the multicolor dots. In this method, since coloring materials used are dyes, the formed images are very sharp and are highly transparent and thus are excellent in reproduction of intermediate colors and in gradation and are comparable with images formed by conventional offset printing or gravure printing. At the same time, this method can form high-quality images comparable with full-color images formed by photography.
In the thermal transfer recording method utilizing the thermal dye sublimation transfer, an increase in printing speed of thermal transfer printers has posed a problem that conventional thermal transfer sheets cannot provide satisfactory print density. Further, higher density and higher sharpness have become required of prints of images formed by thermal transfer. To meet this demand, various attempts have been made to improve thermal transfer sheets and thermal transfer image-receiving sheets which receive sublimable dyes transferred from the thermal transfer sheets to form images. For example, an attempt to improve the sensitivity in transfer at the time of printing has been made by reducing the thickness of the thermal transfer sheet. This, however, poses a new problem that cockling occurs due to heat, pressure or the like applied at the time of the production of the thermal transfer sheet or at the time of thermal transfer recording and, in some cases, breaking of the thermal transfer sheet occurs.
Further, as described in patent document 1, an attempt to improve the print density and the sensitivity in transfer at the time of printing has been made by increasing the dye/resin binder ratio in the dye layer of the thermal transfer sheet. In this case, however, during storage in a wound state, the dye is transferred onto the heat resistant slip layer provided on the backside of the thermal transfer sheet, and, at the time of rewinding, the dyes transferred onto the heat resistant slip layer are retransferred onto dye layers of other colors or the like (a kick back phenomenon). When the contaminated layers are thermally transferred onto an image receiving sheet, hue different from a designated one is provided, or otherwise the so-called “smudge” occurs. To overcome the above problem, a proposal on a thermal transfer printer rather than the thermal transfer sheet side has been made. In this proposal, in thermal transfer at the time of image formation, high energy is applied in a thermal transfer printer. In this case, however, fusing of the dye layer to the receptive layer, that is, the so-called “abnormal transfer,” is likely to occur. When a large amount of a release agent is added to the receptive layer for abnormal transfer prevention purposes, blurring, smudge and other unfavorable phenomena of the image occur.
Further, a proposal has also been made in which the maximum transfer density is enhanced by selecting a resin binder having a relatively low glass transition temperature for a dye layer in a thermal transfer sheet. In this case, however, the binder is disadvantageous in that the release of the dye occurs even upon exposure to a relatively low level of energy and, as a result, the transfer density is higher than the set value also in the highlight part in printing, resulting in a deterioration in reproduction of thermally transferred images. Patent document 2 describes that a binder resin containing not less than 90% by weight of a polyvinyl butyral resin, in which the molecular weight range and the glass transition temperature range have been specified and, further, the content of the vinyl alcohol part has been specified, is used as a component of the dye layer. Even when this thermal transfer sheet is used, however, the maximum transfer density is not on a satisfactory level.
Patent document 1: Japanese Patent Laid-Open No. 295083/1996
Patent document 2: Japanese Patent Publication No. 29504/1995